The Power of the High Pass Filter

If you're
a graphic artist working in the game industry, it is almost certain that
at least at some point of your career you have had to deal with tiling
textures in Photoshop. Textures, that must not only tile seamlessly, but
also look good even when tiled tens of times over. Ever too often, your
brick wall may tile seamlessly at the edges, but something, just something
there doesn't work: when the texture repeats many times over, you see
annoying repetition, covering the distant hills with an unnatural raster-like
pattern. You may realize that there's a cloudy, darker area that needs
to be removed, try to Dodge it a little, and try again. Okay, the dark
blotch is now gone-but wait, now there's a bright area in the other corner!
And so it goes, driving you nuts.

More recently,
using digital photo material for textures has also been an issue. Taking
the pictures is an art in itself, but working textures out of them is
often where the most hair-pulling takes place. Ever had to take pictures
of a concrete wall in a dark underground garage, using a flash? The picture
will end up with a prominent highlight in the middle, which may look good
in a completely non-lit environment, but not when rendered with lightmaps.
How do you deal with it? Creating gradual selections by using gradient
tools in the Quick Mask mode may do the trick, but not perfectly, and
not without a lot of trouble.

FIGURE 1. A brick wall with an annoyingly repeating texture.

The third
area that has traditionally been somewhat tricky, is making detail textures
for today's 3D accelerators. These, if any, must tile perfectly unrepeated,
or any surface using them may look even worse with one than without.

For all
these problems, there is an incredibly handy little filter that most people
seem to ignore. It is called the High Pass filter.

FIGURE 2: A photograph of a wall texture, taken in dark with a flash.

Underrated, Yet
Powerful

If you've
ever explored the secluded Other.. section in Photoshop's Filters menu,
you may have seen the High Pass filter. You might have tried it, and seen
how it pales and flattens the image. What possible use could there be
for such a filter, except for some odd postmodern weathering effects?
You may have looked at Photoshop's online help, or even some tutorial
books on the subject, but none of them do much about explaining what the
filter does beyond its conventional uses - that is, "extracting detail
out of continuous-tone images prior to posterizing". This isn't something
a graphic artist working in game industry often needs to do.

I once decided
to experiment a bit further with High Pass, however, and realized its
true potential - and nowadays, I couldn't live without it.

First, I
am going to tell you something about the world of signals and waveforms.
Strange it may sound, but pictures and sound have much more in common
than many people think.